Physiology of vision

Question 1

Perception of light

Answer 1

• photons of shorter wavelengths have more energy
• visible light occupies small portion of EM spectrum
• light travels in straight lines
• sunlight contains all wavelengths
• objects absorb some wavelengths and reflect others, they present as the colour reflected

Question 2

Lens system

Answer 2

• in vertebrates
• focuses light from objects in environment onto the retina at rear of eye
• retina is where sensory cells are located
• transmits information to the brain

Question 3

Ciliary bodies

Answer 3

thickened anterior parts of choroid (between sclera and retina)
- produce aqueous humour

Question 4

Lens

Answer 4

adjustable focus mediated by ciliary bodies

Question 5

Optics of the eye

Answer 5

• cornea refracts light entering the eye
• parallel light rays hit biconcave lens, refracted to a point beyond the lens called PRINCIPLE FOCUS
• principle focus distance is the distance between the lens and the principle focus
• dioptres are unit of refractive measurement

Question 6

Lens; focusing images

Answer 6

• Further the object is, the smaller the distance between the lens and the focal point because light rays are parallel
• Closer object’s refracted light rays are not parallel so distance increases
• Increases curvature of lens (convexity) decreases distance

Question 7

Accommodation to focus images

Answer 7

Process by which the curvature of the lens is increased to focus on a near object
- ciliary bodies contract
- distance between ciliary bodies decrease
- lens ligaments relax
- active process

Question 8

Ciliary bodies role in accomodation

Answer 8

• Lens is attached to ciliary body via zonulas
• Zonulas are made of inelastic fibres
• Ciliary bodies contract, tension released on zonulas
• more convex shape formed

Question 9

Iris

Answer 9

• pigmented and opaque
• contains constrictors of the pupil
• and dilators of the pupil
• controls amount of light let into the eye

Question 10

Quantity of light control

Answer 10

• Iris; smooth muscle in mammals, striated in birds and reptiles
• autonomic innervation
• radial muscle fibres dilate pupil
• circular muscle fibres constrict pupil

Question 11

Types of pupil shapes

Answer 11

• Vertical slit; in some carnivores. Allows greater light control. Enhances distance judgement
• Rectangle; in herbivores. Sheilds eye from sunlight, greater span of vision for predators

Question 12

Vitreous humour

Answer 12

a gelatinous substance between the lens and the retina
- not continually replaced
- retains globe shape

Question 13

Aqueous humour

Answer 13

a clear liquid that nourishes the cornea and the lens
* Bathes lens and cornea providing nutrients
* Drains into venous plexus at the filtration angle
* Production and drainage must remain balanced

Question 14

Schlemm’s canal

Answer 14

at the junction between the iris and the cornea
- drains the aqueous humour

Question 15

Sclera

Answer 15

outer layer of eye
- tough connective tissue
- merges with cornea

Question 16

Cornea

Answer 16

transparent layer at front, for physical protection, refraction of light and highly sensitive

Question 17

Choroid

Answer 17

vascularised middle layer of the eye

Question 18

Retina

Answer 18

• innermost layer containing sensory cells and
  neurons

Question 19

Cell types in retina

Answer 19

• 2x sensory cell types; rods and cones
• rods; sensitive for vision in low light
• cones; colour vision
• bipolar cells; connect sensory cells and ganglion cells
• horizontal cells; connect groups of sensory cells
• amacrine cells; forms connections between ganglion cells

Question 20

Layers of retina

Answer 20

3x layers
- Outermost layer; closest to choroid. Rods and cones
- Middle layer; conatins the interneuron cell types (bipolar, horizontal and amacrine)
- Innermost layer; ganglion cells that are connected to the optic nerve

Question 21

The Optic Disc

Answer 21

Axons of the ganglion cells converge to form the optic nerve
- Optic disc is the point where the optic nerve leaves the eye & blood vessels enter
- Blind spot

Question 22

Macula Lutea & Fovea Centralis

Answer 22

• Macula lutea; yellow area near posterior pole marks location of fovea centralis
• Fovea centralis; rod-free cone-packed area
• NO blood vessels
• Maximum visual accuity

Question 23

Retinal vessels

Answer 23

• supply the bipolar and ganglion cells

Question 24

Choroid plexus

Answer 24

• supplies the rods and cones

Question 25

Segments of rods and cones

Answer 25

- Outer segment; modified cilia that form saccules and discs containing photosensitive compounds - Inner segments; contains a nuclear region rich in mitochondria - Synaptic zone; synapses with bipolar or horizontal cells

Question 26

Rods vs. cones

Answer 26

- Cones have narrower tip compared to base - Rods have long outer segments of even thickness - Rods are more numerous usually and more sensitive - Cones are more abundant near fovea - Both have photopigments

Question 27

Photopigments

Answer 27

- Photoreceptor cells hyperpolarise in response to light - Light removes cGMP so cyclic nucleotide-gated ion channels are closed - Rods and cones contain; Retinal and Opsins - cis-retinal is converted to all-trans-retinal in light - this activates transducin to convert cGMP to GMP

Question 28

Retinal

Answer 28

- aldehyde of vitamin A - 11-cis-retinal is converted to all-trans-retinal in light

Question 29

Opsins

Answer 29

- modifies retinals sensitivity to differing wavelengths of light

Question 30

Eye movements

Answer 30

-Saccadic eye movements (rapid) – take lots of snapshots which the brain interprets as a stable image - Vestibulo-ocular reflex enables maintenance of an image despite rapid head movement

Question 31

Physiological nystagmus

Answer 31

When image extends past range of ocular motion a correcting saccade brings the eye back

Question 32

Pathological nystagmus

Answer 32

When the eye moves and saccades when the head is still indicating a problem with the vestibular system

Question 33

Colour perception

Answer 33

- Depends on the different types of opsins and different cone types - Can be di, tri... chromats, refering to number of types of cones

Question 34

Visual Pathways

Answer 34

- the right side of each retina collects light from the left visual field and vice versa - medial ganglia cross over at optic chiasm - lateral ganglia do NOT cross over - contralateral processing; both sides of the brain process info from both eyes -

Question 35

Retinal Convergence

Answer 35

- many rods can converge to just a single ganglion cells - individual cones can synapse with individual ganglion cells

Question 36

Vision processing and Ganglion cells

Answer 36

- ganglion cells only detect a difference in light intensity between the centre and periphery of their receptive fields